WELDING PROCESSES

Welding Processes

Welding is not new. The earliest known form of welding, called forge welding, dates back to the year 2000 B.C. Forge welding is a primitive process of joining metals by heating and hammering until the met-als are fused (mixed) together. Although forge welding still exists, it is mainly limited to the blacksmith trade.

Today, there are many welding processes available and used in modern metal fabrication and repair. This list, published by the American Welding Society (AWS), shows the official abbreviations for each process. For example, RSW stands for resistance spot welding. Shielded metal arc welding (SMAW) is an arc-welding process that fuses (melts) metal by heating it with an electric arc created between a covered metal electrode and the metals being joined. Of the welding processes , shielded metal arc welding, called stick welding, is the most common welding process. The primary differences between the various welding processes are the methods by which heat is generated to melt the metal. Once you understand the theory of welding, you can apply it to most welding processes.

The most common types of welding are oxyfuel gas welding (OFW), arc welding (AW), and resistance welding (RW). As a steelworker, your primary concern is gas and arc welding. The primary difference between these two processes is the method used to generate the heat.

Gas Welding

One of the most popular welding methods uses a gas flame as a source of heat. In the oxyfuel gas welding process , heat is produced by burning a combustible gas, such as MAPP (methylacetylene-propadi-ene) or acetylene, mixed with oxygen. Gas welding is widely used in maintenance and repair work because of the ease in transporting oxygen and fuel cylinders. Once you learn the basics of gas welding, you will find the oxyfuel process adaptable to brazing, cutting, and heat treating all types of metals.

Arc Welding

Arc welding is a process that uses an electric arc to join the metals being welded. A distinct advantage of arc welding over gas welding is the concentration of heat. In gas welding the flame spreads over a large area, sometimes causing heat distortion. The concentration of heat, characteristic of arc welding, is an advantage because less heat spread reduces buckling and warping. This heat concentration also increases the depth of penetration and speeds up the welding operation; therefore, you will find that arc welding is often more practical and economical than gas welding.

Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding is performed by striking an arc between a coated-metal electrode and the base metal. Once the arc has been established, the molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to forma sound joint. This process is known as fusion. The coating from the electrode forms a covering over the weld deposit, shielding it from contamination; therefore the process is called shielded metal arc welding. The main advantages of shielded metal arc welding are that high-quality welds are made rapidly at a low cost.

Gas Shielded Arc Welding The primary difference between shielded metal arc welding and gas shielded arc welding is the type of shielding used. In gas shielded arc welding, both the arc and the molten puddle are covered by a shield of inert gas. The shield of inert gas prevents atmospheric contamination, thereby producing a better weld. The primary gases used for this process are helium, argon, or carbon dioxide. In some instances, a mixture of these gases is used. The processes used in gas shielded arc welding are known as gas tungsten arc welding (GTAW)

GTAW and gas metal arc welding (GMAW). You will also hear these called “TIG” and “MIG.” Gas shielded arc welding is extremely useful because it can be used to weld all types of ferrous and nonferrous metals of all thicknesses.

Now that we have discussed a few of the welding processes available, which one should you choose? There are no hard-and-fast rules. In general, the control-ling factors are the types of metal you are joining, cost involved, nature of the products you are fabricating, and the techniques you use to fabricate them. Because of its flexibility and mobility, gas welding is widely used for maintenance and repair work in the field. On the other hand, you should probably choose gas shielded metal arc welding to repair a critical piece of equipment made from aluminium or stainless steel.

No matter what welding process you use, there is some basic information you need to know. The remainder of this chapter is devoted to this type of information. Study this information carefully because it allows you to follow welding instructions, read welding symbols, and weld various types of joints using the proper welding techniques

Heat Colours for Steel

You are probably familiar with the term red-hot as applied to steel. Actually, steel takes on several colours and shades from the time it turns a dull red until it reaches a white heat.

During hardening, normalizing, and annealing, steel is heated to various temperatures that produce colour changes. By observing these changes, you can determine the temperature of the steel. As an example, assume that you must harden a steel part at 815 Celsius or 1500 Fahrenheit. Heat the part slowly and evenly while watching it closely for any change in colour. Once the steel begins to turn red, carefully note each change in shade. Continue the even heating until the steel is bright red; then quench the part.

The success of a heat-treating operation depends largely on your judgment and the accuracy with which you identify each colour with its corresponding temperature. From a study, you can see that close observation is necessary. You must be able to tell the difference between faint red and blood red and between dark cherry and medium cherry. To add to the difficulty, your conception of medium cherry may differ from that of the person who prepared the table. For an actual heat-treating operation, you should get a chart showing the actual colours of steel at various temperatures.

Manual Shielded Metal-Arc Welding

Arc welding provides you the ability to join two metals by melting them with an arc generated between a coated-metal electrode and the base metal. The temperatures developed by the arc can reach as high as 10000°F. The arc energy is provided by a power source that generates either direct or alternating current. The electrodes that carry the current produce a gas that shields the arc from the atmosphere and supplies filler metal to develop the weld shape.

ARC-WELDING EQUIPMENT A wide variety of welding equipment is available, and there are many differences between the makes and models of the equipment produced by the manufacturers. However, all types of arc-welding equipment are similar in their basic function of producing the high-amperage, low-voltage electric power required for the welding arc. In this discussion, we are primarily concerned with the typical items of arc-welding equipment, rather than the specific types. For specific information about the equipment your battalion or duty station has available, consult the manufacturer’s instruction manual.

The basic parts of a typical shielded metal-arc welding outfit include a welding machine, cables, electrode holder (stinger), and electrodes. The steelworker also requires a number of accessories that include a combination chipping hammer and wire brush, welding table (for shop work), C-clamps, and protective apparel.

Before we discuss the different types of welding machines, you must first have a basic knowledge of the electrical terms used with welding.

Electrical Terms : Many terms are associated with arc welding. The following basic terms are especially important.

ALTERNATING CURRENT.— Alternating current is an electrical current that has alternating negative and positive values. In the first half-cycle, the current flows in one direction and then reverses itself for the next half-cycle. In one complete cycle, the current spends 50 percent of the time flowing one way and the other 50 percent flowing the other way. The rate of change in direction is called frequency, and it is indicated by cycles per second. In the United States, the alternating current is set at 60 cycles per second.

AMPERE.— Amperes, sometimes called “amps,” refers to the amount of current that flows through a circuit. It is measured by an “amp” meter.

CONDUCTOR.— Conductor means any material that allows the passage of an electrical current.

CURRENT.— Current is the movement or flow of an electrical charge through a conductor.

DIRECT CURRENT.— Direct current is an electrical current that flows in one direction only.

ELECTRICAL CIRCUIT.— Electrical circuit is the path taken by an electrical current flowing through a conductor from one terminal of the source to the load and returning to the other terminal of the source.

POLARITY.— Polarity is the direction of the flow of current in a circuit. Since current flows in one direction only in a dc welder, the polarity becomes an important factor in welding operations.

RESISTANCE.— Resistance is the opposition of the conductor to the flow of current. Resistance causes electrical energy to be changed into heat.

VOLT.— A volt is the force required to make the current flow in an electrical circuit. It can be compared to pressure in a hydraulic system. Volts are measured with a volt meter.

Power Source The power source used in arc welding is called a welding machine or a welder. Three basic types of welding machines are presently in use: motor-generators, transformers, and rectifiers.

Gas Tungsten-Arc Welding—GTAW

Gas tungsten-arc welding is basically a form of arc welding; however, in gas tungsten-arc welding, the electrode is used only to create the arc. The electrode is not consumed in the weld as in the shielded metal-arc process. The gas tungsten-arc welding process generally produces welds that are far superior to those produced by metallic arc welding electrodes. Especially useful for welding aluminium, it also may be used for welding many other types of metals. The GTA process is most effective for joining metals up to 1/8 inch thick, although you can use it to weld thicker material.

As shown in figure 8-2, the basic GTA process involves an intense arc between the base metal and a tungsten electrode. The arc, the electrode, and the weld zone are surrounded by an inert gas (usually either helium or argon or a mixture of the two) that displaces the air and eliminates the possibility of weld contamination by the oxygen and nitrogen present in the atmosphere. The tungsten electrode has a high melting point that makes it virtually no consumable.

Specific advantages of gas tungsten-arc welding include the following:

Welding can be done in all positions.

The weld is usually equal to the base metal in composition.

Flux is not used; therefore, finished welds do not require cleaning of corrosive residue.

Smoke or fumes are not present to obscure vision; therefore, you can easily see the welding process.

Distortion of the base metal is minimal because the heat is concentrated in a small area.

No splatter is produced because metal is not transferred across the arc.

...MORE ABOUT WELDING TYPES

Welders use many types of welding equipment set up in a variety of positions, such as flat, vertical, horizontal, and overhead. They may perform manual welding, in which the work is entirely controlled by the welder, or semiautomatic welding, in which the welder uses machinery, such as a wire feeder, to perform welding tasks.

There are about 100 different types of welding. Arc welding is the most common type. Standard arc welding involves two large metal alligator clips that carry a strong electrical current. One clip is attached to any part of the workpiece being welded. The second clip is connected to a thin welding rod. When the rod touches the workpiece, a powerful electrical circuit is created. The massive heat created by the electrical current causes both the workpiece and the steel core of the rod to melt together, cooling quickly to form a solid bond. During welding, the flux that surrounds the rod’s core vaporizes, forming an inert gas that serves to protect the weld from atmospheric elements that might weaken it. Welding speed is important. Variations in speed can change the amount of flux applied, weakening the weld, or weakening the surrounding metal by increasing heat exposure.

Two common but advanced types of arc welding are Tungsten Inert Gas (TIG) and Metal Inert Gas (MIG) welding. TIG welding often is used with stainless steel or aluminum. While TIG uses welding rods, MIG uses a spool of continuously fed wire, which allows the welder to join longer stretches of metal without stopping to replace the rod. In TIG welding, the welder holds the welding rod in one hand and an electric torch in the other hand. The torch is used to simultaneously melt the rod and the workpiece. In MIG welding, the welder holds the wire feeder, which functions like the alligator clip in arc welding. Instead of using gas flux surrounding the rod, TIG and MIG protect the initial weld from the environment by blowing inert gas onto the weld.

Like arc welding, soldering and brazing use molten metal to join two pieces of metal. However, the metal added during the process has a melting point lower than that of the workpiece, so only the added metal is melted, not the workpiece. Soldering uses metals with a melting point below 800 degrees Fahrenheit; brazing uses metals with a higher melting point. Because soldering and brazing do not melt the workpiece, these processes normally do not create the distortions or weaknesses in the workpiece that can occur with welding. Soldering commonly is used to join electrical, electronic, and other small metal parts. Brazing produces a stronger joint than does soldering, and often is used to join metals other than steel, such as brass. Brazing can also be used to apply coatings to parts to reduce wear and protect against corrosion.

Skilled welding, soldering, and brazing workers generally plan work from drawings or specifications or use their knowledge of fluxes and base metals to analyze the parts to be joined. These workers then select and set up welding equipment, execute the planned welds, and examine welds to ensure that they meet standards or specifications. They are even examining the weld while they’re welding. By observing problems with the weld, they compensate by adjusting the speed, voltage, amperage, or feed of the rod. Highly skilled welders often are trained to work with a wide variety of materials in addition to steel, such as titanium, aluminum, or plastics. Some welders have more limited duties, however. They perform routine jobs that already have been planned and laid out and do not require extensive knowledge of welding techniques.

Automated welding is used in an increasing number of production processes. In these instances, a machine or robot performs the welding tasks while monitored by a welding machine operator. Welding, soldering, and brazing machine setters, operators, and tenders follow specified layouts, work orders, or blueprints. Operators must load parts correctly and constantly monitor the machine to ensure that it produces the desired bond.

The work of arc, plasma, and oxy-gas cutters is closely related to that of welders. However, instead of joining metals, cutters use the heat from an electric arc, a stream of ionized gas (plasma), or burning gases to cut and trim metal objects to specific dimensions. Cutters also dismantle large objects, such as ships, railroad cars, automobiles, buildings, or aircraft. Some operate and monitor cutting machines similar to those used by welding machine operators. Plasma cutting has been increasing in popularity because, unlike other methods, it can cut a wide variety of metals, including stainless steel, aluminum, and titanium.